The band-edge photoluminescence (PL) intensity of etched n-CdSe single crystals is reversibly enhanced in the presence of nitrogen-saturated toluene solutions of trans-IrCl(CO)(PPh(3))(2), Vaska’s complex (1), relative to nitrogen-saturated toluene alone. The PL enhancement is indicative of adsorption of a Lewis base onto the n-CdSe crystals. When the toluene solution of 1 is saturated with carbon monoxide, repetition of these experiments produces larger reversible PL enhancements; when the same solution is saturated with oxygen, large reversible quenching is observed relative to toluene. These PL intensity changes are attributed to binding to the surface of 1 . CO and 1 . O-2, the CO and O-2 adducts of 1, respectively. The concentration-dependent PL changes are well fit by the Langmuir adsorption isotherm model; responses to 1 and 1 . CO are characterized by similar binding constants K of nearly 10(5) M(-1), while K for 1 . O-2 is 10(7) M(-1). In the solid state, 1 is known to bind CO and O-2 reversibly. When the CdSe surface is coated with thin films of 1, evaporated from toluene solution, CO and O-2 gases can be detected through reversible enhancement and quenching, respectively, of CdSe PL intensity relative to a gaseous nitrogen ambient; neither gas affects the PL intensity of uncoated CdSe samples appreciably. Films of 1 coated onto NaCl plates or CdSe crystals yield binding constants for CO from IR spectral changes that are experimentally indistinguishable from those obtained by PL measurements, similar to 10(2) M(-1). In contrast, the binding constant for O-2, determined from IR spectral changes to be similar to 10(2) M(-1), is roughly an order of magnitude lower than that estimated from PL changes, evidencing a roughly 10-fold enhancement in binding affinity at the semiconductor-film interface for 1 toward O-2 relative to the bulk film environment. Aspects of the coordination chemistry of 1 that may account for these electrooptical effects are discussed.